Boss Event

In EliteMobs (EM), in the nether and the overworld, randomly, a EM boss can spawn in the world. You can click to track it in your bossbar.

Example of a boss spawning message in chat

When you track it, it looks like this:

On bedrock, since you can't click on it in the chat, just use /em -> Boss Tracking (If the gui appears as a book, just use /em alts to turn it into a chest gui)

/em -> Boss Tracking menu

Random EM boss can be pretty hard to fight if you are new.

Random boss (Every boss is different)

Those boss give different Elitemobs loots, these loots can be different for every boss.